1. Field of the Invention
This invention relates to an internal thermal protector for electric motors used in hermetic electrically-driven compressors, and more particularly to such an internal thermal protector for protecting three-phase motors against burnout.
2. Description of the Prior Art
As conventional thermal protectors of the above-described type, Japanese Patent Publication No. 56-130040A discloses a thermally responsive switch and Japanese Patent Publication No. 62-88232A discloses a thermally responsive snap action switch. These disclosed protectors are mounted on single-phase motors and comprise a thermally responsive element such as a bimetal having a central dish-shaped or otherwise, curved portion formed by drawing. The thermally responsive element has one end carrying a movable contact engaged with and disengaged from a fixed contact and the other end secured to an elastic plate. In determination of a contact pressure between the movable and fixed contacts and an operating temperature of the protector, both ends of the thermally responsive element are abutted against the fixed contact and a support respectively in an inoperative condition of the protector. In this state, a predetermined ressure is applied to a central convex portion of the thermally responsive element, so that the contact pressure and the operating temperature are determined. In these protectors, the thermally responsive element and parts around the element heat up when a current flows between terminals. The thermally responsive element changes its state of curvature with snap action when an ambient temperature is increased because of an abnormal condition of the motor or when the motor is in an overheat condition due to an overcurrent. Consequently, the movable contact secured to the one end of the thermally responsive element is disengaged from the fixed contact, thereby cutting off a current path.
Japanese Patent Publication Nos. 57-34623A, 1-105435A and 6-96649A disclose thermal protectors for a three-phase motor respectively. Each of these protectors comprises a generally circular thermally responsive element having a central dish-shaped or otherwise, curved portion formed by drawing. Two movable contacts and an elastic member are secured to the thermally responsive element so that points of securement form an imaginary substantially equilateral triangle. In determination of the contact pressure and the operating temperature of the protector, the movable contacts and the elastic member are abutted against fixed contacts and a support respectively. In this state, a predetermined pressure is applied to a central portion of the thermally responsive element which is convex at an ordinary temperature, so that the contact pressure and the operating temperature are determined. These three-phase motor protectors are connected to a neutral point of windings of a three-phase motor arranged in a wye configuration. Upon occurrence of an abnormal condition of the motor, the thermally responsive element changes its state of curvature in the same manner as described above about the thermal protectors for the single-phase motors, thereby opening the neutral point to cut off interphase current paths.
In the thermal protectors for the three-phase motors, the current is caused to flow through the thermally responsive element so that heat generated by the thermally responsive element is primarily used to operate the same. The two movable contacts and the elastic member are secured to the generally circular thermally responsive element so that points of securement form an imaginary substantially equilateral triangle. The three-phase thermal protectors are formed into a relatively flat configuration and has approximately equal longitudinal and lateral dimensions.
In each of the above-described motor protectors, the thermally responsive element carries the movable contact on the free end thereof and has the other end connected to the elastic plate and serving as a supported end, so that an excessive amount of stress is prevented from concentrating on the supported end. The elastic plate also serves as a heating element for compensating the heat generated by the thermally responsive element. However, in the case where the motor has a large rated current or where an operating temperature of the thermally responsive element is set at a large value, the elastic plate causes an annealing effect when serving as the heating element with its temperature being increased. Consequently, the performance of the elastic plate as an elastic member is reduced. Thus, the elastic plate cannot be used in the thermal protector when it is used for the motors of the high output type. A rigid heating element would be considered to be used instead of the elastic plate for the solution of the above-described problem. In this case, however, there is the possibility that an excessive stress may concentrate on the supported end of the thermally responsive element, resulting in variations in the operating temperature thereof. This entails a problem.
The elastic plate can be twisted even when the thermally responsive element is inclined relative to the fixed contacts to some extent in assembling. Consequently, the contact pressures can be equilibrated between the two pairs of contacts without being adversely affected by the inclination of the thermally responsive element in the three-phase thermal protector using the elastic plate. On the other hand, the above-described equilibrating adjustment cannot be carried out when the thermally responsive element is supported by a rigid member. Accordingly, separate contact pressure adjusting mechanisms need to be provided for the respective pairs of contacts, or the two pairs of contacts need to be assembled with such a high-level accuracy that the two pairs of contacts are simultaneously engaged and disengaged and that the contact pressures are rendered equal. This entails another problem.
The conventional three-phase thermal protectors include the generally circular thermally responsive element and are formed to have a relatively flat configuration and approximately equal longitudinal and lateral dimensions, as described above. Particularly when the protector is bound to end turns of the motor winding with strings, the protector thus mounted becomes unstable since the protector has a larger width than the end turns of the winding. The size of the electrically-driven compressor has recently been reduced and the diameter of the motor used in the compressor has also been reduced accordingly. The overall size of the conventional protector needs to be reduced when the width thereof is rendered smaller than that of the end turns of the winding. However, this reduces a current capacity of the protector and renders the manufacture of the protector difficult. This entails further another problem.
Furthermore, a work for attaching an insulating covering to the conventional thermal protector for the insulation thereof is troublesome since the protector is formed to have a relatively flat configuration and approximately equal longitudinal and lateral dimensions. A receptacle of the protector usually serves as an electrically conducting section. A failure such as short circuit occurs when the receptacle is brought into contact with another electrically conducting part or member. Accordingly, the thermal protector needs to be covered by the insulating covering such as a polyester tube. The polyester tube having an inner circumference larger than an outer circumference of the protector is put onto the protector to cover the latter. The tube is fixed to the surface of the protector by heat shrinkage. Since a polyester film does not have a large coefficient of heat shrinkage, the difference between the outer circumference of the protector and the inner circumference of the polyester tube is small. Accordingly, the generally cylindrical polyester tube needs to be deformed to a large extent in order to be fitted with the protector when the protector is formed to have a relatively flat configuration and approximately equal longitudinal and lateral dimensions. Thus, positioning the polyester tube is difficult and the work for attaching the tube to the protector is troublesome. This entails further another problem.